This invention relates to catalysts for production of unsaturated aldehyde and unsaturated carboxylic acid and to a process for producing unsaturated aldehyde and unsaturated carboxylic acid. More particularly, the invention relates to improved catalysts for producing unsaturated aldehyde and unsaturated carboxylic acid at high yield and with stability over a prolonged period, by vapor phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether; and also to a process for producing unsaturated aldehyde and unsaturated carboxylic acid using the improved catalysts.
A number of improved catalysts have been proposed for high efficiency production of unsaturated aldehyde and unsaturated carboxylic acid by vapor phase catalytic oxidation reaction of propylene, isobutylene, t-butanol or methyl-t-butyl ether. The most of those proposed catalysts contain molybdenum, bismuth and iron as their chief components.
However, those catalysts still are subject to problems which need to be solved, in respect of yields of unsaturated aldehyde and unsaturated carboxylic acid and the catalyst life. Molybdenum in a catalyst is apt to sublime, which induces irreversible degradation in the catalytic activity. The intended oxidation reaction is extremely exothermic and in the catalyst layers, in particular, at the local abnormally high temperature zone which is referred to as hot spot, molybdenum sublime vigorously. Particularly in heavy load operation aiming at high productivity, naturally the heat accumulation at the hot spot increases to prolong the period during which the catalyst is used under high temperatures. Considering these factors, catalysts having high activity and exhibiting stable performance over a long period are in demand.
On the other hand, various proposals have been also made as to complex oxides whose essential components are cerium and zirconium. Most of these complex oxides are known as additive components to waste gas-combustion catalysts, but it is entirely unknown that such complex oxides whose essential components are cerium and zirconium exhibit effective catalytic activity in production of unsaturated aldehyde and unsaturated carboxylic acid through vapor phase catalytic oxidation of propylene, isobutylene, t-butanol or methyl-t-butyl ether, when used in combination with molybdenum-bismuth-iron-containing catalysts.
One of the objects of the present invention is to provide catalysts which are useful in production of unsaturated aldehyde and unsaturated carboxylic acid at high yield.
A further object of the present invention is to provide catalysts for unsaturated aldehyde and unsaturated carboxylic acid production, which have long catalyst life and enable stable operation over prolonged period.
Still another object of the present invention is to provide catalysts for unsaturated aldehyde and unsaturated carboxylic acid production which enable stable operation over prolonged period, even under heavy load operation aiming at high productivity.
An additional object of the present invention is to provide a process for producing unsaturated aldehyde and unsaturated carboxylic acid at high yield and with stability over prolonged period, using the above catalysts.
We have discovered that a composition, in which a complex oxide known as catalyst for unsaturated aldehyde and unsaturated carboxylic acid production, containing molybdenum, bismuth and iron as the essential components, is combined with a complex oxide whose essential components are cerium and zirconium, exhibits high catalytic activity in the intended reaction and excellent stability; and that the use of such a composition as a catalyst in said reaction accomplishes the above objects.
Thus, according to the invention, as a catalyst for producing unsaturated aldehyde and unsaturated carboxylic acid through oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas at vapor phase, a complex oxide composition characterized by comprising
(A) a complex oxide containing as essential components molybdenum, bismuth and iron, which is known per se as a catalyst for said vapor phase catalytic oxidation reaction, and
(B) a complex oxide containing cerium and zirconium as the essential components, is provided.
According to the present invention, there is also provided, as a catalyst for producing unsaturated aldehyde and unsaturated carboxylic acid through oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas at vapor phase, a complex oxide composition which is characterized by having a composition expressed by the following general formula (3):
MoaWbBicFedCeeZrfAgBhCiDjEkGlOxxe2x80x83xe2x80x83(3) 
(wherein Mo is molybdenum; W is tungsten: Bi is bismuth;
Fe is iron; Ce is cerium; Zr is zirconium; A is at least an element selected from nickel and cobalt; B is at least an element selected from alkali metals and thallium; C is at least an element selected from alkaline earth metals; D is at least an element selected from phosphorus, tellurium, antimony, tin, lead, niobium, manganese, arsenic and zinc; E is at least an element selected from silicon, aluminium and titanium; G is at least an element selected from the group consisting of lanthanoide series except cerium; yttrium, copper, indium, chromium and germanium; and O is oxygen; a, b, c, d, e, f, g, h, i, j, k, l and x denote the atomic ratios of Mo, W, Bi, Fe, Ce, Zr, A, B, C, D, E, G and O, respectively, and where a is 12, b is 0-10, c is 0.1-10, d is 0.1-20, e is 0.01-30, f is 0.01-42, g is 2-20, h is 0.001-10, i is 0-10, j is 0-4, k is 0-30, l is 0-7, and x is a numerical value determined by degree of oxidation of each of the elements)
and the cerium and zirconium therein forming a complex oxide.
According to the invention, furthermore, there is provided a process for producing unsaturated aldehyde and unsaturated carboxylic acid through vapor phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas, in the presence of a catalyst, the process being characterized in that it uses the above-defined complex oxide composition as the catalyst.
Catalyst (I) for unsaturated aldehyde and unsaturated carboxylic acid production according to the invention is a complex oxide composition which is characterized by comprising
(A) a complex oxide containing as essential components molybdenum, bismuth and iron, which is known per se as a catalyst for producing unsaturated aldehyde and unsaturated carboxylic acid through said vapor phase catalytic oxidation reaction of propylene, isobutylene, t-butanol and methyl-t-butyl ether, and
(B) a complex oxide containing cerium and zirconium as the essential components.
Catalyst (II) for unsaturated aldehyde and unsaturated carboxylic acid production according to another embodiment of the present invention is a complex oxide composition which is characterized by having the composition as expressed by above general formula (3), in which cerium and zirconium form a complex oxide.
First, the Catalyst (I) shall be explained. The component (A) corresponds to a catalyst containing molybdenum, bismuth and iron as the essential components, which is known as a catalyst for unsaturated aldehyde and unsaturated carboxylic acid production by vapor phase oxidation reaction of propylene, isobutylene, t-butanol or methyl-t-butyl ether. While any of known catalysts containing molybdenum, bismuth and iron as the essential components can be used as the component (A), those preferred are expressed by the following general formula (1):
MoaWbBicFedAeBfCgDhEiOxxe2x80x83xe2x80x83(1) 
(wherein Mo is molybdenum; W is tungsten; Bi is bismuth;
Fe is iron; A is at least an element selected from nickel and cobalt; B is at least an element selected from alkali metals and thallium; C is at least an element selected from alkaline earth metals; D is at least an element selected from phosphorus, tellurium, antimony, tin, lead, niobium, manganese, arsenic and zinc; E is at least an element selected from silicon, aluminium and titanium;
and 0 is oxygen; a, b, c, d, e, f, g, h, i and x denote the atomic ratios of Mo, W, Bi, Fe, A, B, C, D, E and O, respectively; and where a is 12, b is 0-10, c is 0.1-10, d is 0.1-20, e is 2-20, f is 0.001-10, g is 0-10, h is 0-4, i is 0-30 and x is determined by degree of oxidation of each of the elements).
Method of preparing those catalysts is subject to no critical limitation, and the catalysts can be prepared by any known method. Kinds of the compounds containing the catalytic elements, which serve as the starting materials, are not critical but any oxides containing the catalytic elements or compounds capable of forming such oxides upon being calcined can be used. As the compounds which form oxides upon calcining, for example, hydroxides, metallic acids, nitrates, carbonates, ammonium salts, acetates and formates may be named. Compounds containing more than one of the elements are also useful. For instance, specific examples of molybdenum-containing compounds include molybdenum trioxide, ammonium paramolybdate, molybdic acid, phosphomolybdic acid and phosphovanadomolybdic acid.
Normally each prescribed amount of those starting compounds containing the component elements are, for example, suitably dissolved in an aqueous medium, heated under stirring, evaporated to dry solid and optionally pulverized to provide the intended component (A).
As the component (B), any complex oxide which contains cerium and zirconium as the essential components can be used. In particular, complex oxides which are expressed by the following general formula (2):
CepZrqFrOyxe2x80x83xe2x80x83(2) 
(wherein Ce is cerium; Zr is zirconium; F is at least an element selected from the group consisting of lanthanoide series except cerium; yttrium, cobalt, nickel, copper, indium, tin, chromium and germanium; and O is oxygen; p, q, r and y denote the atomic ratios of Ce, Zr, F and O, respectively, p and q being optional numbers not including 0, r being a number satisfying the relationship 0xe2x89xa6r/(p+q) less than 0.1, and y being a number determined by degree of oxidation of each of the elements)
are conveniently used. More specifically, when p=1, 0.01xe2x89xa6qxe2x89xa699, 0xe2x89xa6r less than 10, preferably 0.05xe2x89xa6qxe2x89xa6r19 and 0-xe2x89xa6r less than 2.
Method of preparing complex oxides containing cerium and zirconium as the essential components is subject to no critical limitation, and they can be prepared by any known method. Kinds of the compounds containing the named elements, which serve as the starting materials, are not critical but any oxides containing the named elements or compounds capable of forming such oxides upon being calcined can be used. As the compounds which form oxides upon being calcined, for example, hydroxides, metallic acids, nitrates, carbonates, ammonium salts, acetates and formates may be named. For example, as a specific example of cerium-containing compound, cerium nitrate may be named.
Of the complex oxides expressed by the general formula (2), those in which cerium oxide and zirconium oxide are at least partially forming a solid solution are conveniently used. In particular, those in which the molar ratio of CeO2/ZrO2 is within a range from 1/99xe2x80x9499/1, preferably 5/95xe2x80x9495/5 are preferred. Furthermore, in the present invention it is essential that the cerium and zirconium are forming a complex oxide. Use of a simple mixture of cerium oxide and zirconium oxide cannot achieve the objects of the present invention.
The component (B) is not limited to complex oxides of cerium and zirconium only, but it may contain the element(s) expressed by the symbol F in general formula (2) in the form of complex oxide. Typical methods for preparation of the component (B) include: (1) mix an aqueous solution of water-soluble cerium salt with that of water-soluble zirconium salt, dry and calcine the same; (2) react cerium oxide with zirconium oxide at solid phase; and (3) impregnate cerium oxide with aqueous solution of water-soluble zirconium salt, dry and calcine the same. The calcining temperature is normally 200-800xc2x0 C., preferably 300-700xc2x0 C. Upon such calcining, complex oxide containing cerium and zirconium is formed.
The ratio of component (B) to component (A) (as converted to oxides) is normally 0.5-30% by weight, preferably 1-20% by weight. When it is too low, the intended effect of adding component (B) cannot be attained, while if it is too high, yield drops as the production amounts of the intended unsaturated aldehyde and unsaturated carboxylic acid reduce and those of CO2 and Co increase.
The catalyst of the present invention can be used by itself or may be supported on inert carriers such as alumina, silica-alumina, silicon carbide, titanium dioxide, magnesium oxide, aluminium sponge and the like. In that occasion inorganic fibers such as glass fiber and various kinds of whiskers, which are generally well known for their effect of improving strength and attrition resistance of catalyst may be added. Also for controlling the catalyst properties with good reproducibility, additives generally known as powder binder such as ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like may be used.
Shape of the catalyst is not critical, which may be any optional form such as pellets, spheres, columns, rings, tablets and the like. Their average diameter is 1-15 mm, preferably 3-10 mm.
Method for preparing the catalyst containing components (A) and (B) is subject to no critical limitation, and any optional method can be used. For example, advancedly prepared powders of the respective components are mixed, optionally using ball mill or the like to effect intimate mixing; or advancedly prepared component (B) is dispersed in component (A) under preparation, at an optional stage.
It is generally preferred for catalyst (I) to be used as molded catalyst as prepared by a process comprising thoroughly mixing the components (A) and (B); imparting to the mixture a desired shape, optionally adding water or the like as a molding aid; and calcining the same in an air stream at 300-600xc2x0 C., preferably 350-550xc2x0 C., for about 1-10 hours, preferably about 2-8 hours.
Catalyst (II) for producing unsaturated aldehyde and unsaturated carboxylic acid according to the present invention has the composition as expressed by the general formula (3) above, in which cerium and zirconium are forming a complex oxide. That is, catalyst (II) contains cerium and zirconium in the form of a complex oxide containing said two elements. A typical method for preparing this catalyst (II) is, similar to the one for preparing catalyst (I), to blend advancedly prepared components (A) and (B). Methods for preparing components (A) and (B), respectively, are same as already described.
Production of unsaturated aldehyde and unsaturated carboxylic acid according to the present invention can be carried out under the conditions normally employed in conventional methods for producing unsaturated aldehyde and unsaturated carboxylic acid from propylene, isobutylene, t-butanol or methyl-t-butyl ether or their mixtures, by vapor phase oxidation reaction, except that a catalyst of the present invention for producing unsaturated aldehyde and unsaturated carboxylic acid is used as the catalyst.
For example, a gaseous mixture comprising 1-10 vol. %, preferably 2-8 vol. % of at least one starting compound selected from the group consisting of propylene, isobutylene, t-butanol and methyl-t-butyl ether; 1-10 times by volume, preferably 1-8 times by volume, of the starting compound of molecular oxygen; and inert gas as a diluent, such as nitrogen, carbon dioxide, steam and the like (use of, in particular, steam is advantageous for improving the yield of the object product, because it inhibits formation of side products), is contacted with a catalyst of the present invention at temperatures ranging from 250 to 450xc2x0 C., preferably from 280 to 420xc2x0 C., under pressures ranging from normal to 10 atmospheres, preferably from normal to 8 atmospheres, and at a space velocity ranging from 300 to 5,000 hrxe2x88x921 (STP), preferably from 500 to 4,000 hrxe2x88x921 (STP).
According to the process of the present invention, acrolein and acrylic acid are produced from propylene; methacrolein and methacrylic acid, from isobutylene; methacrolein and methacrylic acid, from t-butanol; and methacrolein and methacrylic acid, from methyl-t-butyl ether. Obviously, it is possible to vary the production ratios of such unsaturated aldehyde and unsaturated carboxylic acid, by suitably changing the reaction conditions. For example, where propylene is used as starting compound, mixtures of acrolein as the major component and acrylic acid as the minor component can be obtained as the product.
The action of the component (B) in the catalyst of the present invention is presumed to be as follows: highly dispersible zirconium oxide inhibits aggregation of cerium oxide, to maintain the latter""s promoting function to favorably absorb and release oxygen during the reaction, and whereby the oxidation reaction of propylene, isobutylene, t-butanol or methyl-t-butyl ether is accelerated, in consequence increasing the catalytic activity. Furthermore, irreversible activity deterioration in the component (A) complex oxide due to overreduction with time is inhibited (i.e., stability of the complex oxide is increased), resulting in prolongation of the catalyst life. Such increased catalytic activity and extended catalyst life inhibit the rise in the reaction temperature with time, to reduce scattering about of molybdenum at the hot spot. Needless to say, the present invention however is not restricted by such theoretical observation.
Use of the catalyst of the present invention in vapor phase oxidation reaction of at least one compound selected from the group consisting of propylene, isobutylene, t-butanol or methyl-t-butyl ether enables production of unsaturated aldehyde and unsaturated carboxylic acid at high yield. Because the catalyst of the present invention has a long life, it enables stable operation over prolonged period. Furthermore, the catalyst of the present invention enables stable operation over prolonged period also for heavy-load operation aiming at high productivity.